S.L. Steam Outboard

Owned by Wally Mounster


The dinghy was built in Hobart in 1968, its clinker planking is King Billy Pine and the ribs are Huon pine. It was pretty shabby when I bought it and I have spent the last month repainting it inside and out.  I have registered it for display at the Wooden Boat Festival which will be held in Hobart  next February.

Engine & Boiler

This project is based on an 8 h.p. two stroke outboard leg and gearbox. The engine is a single cylinder, double acting type with a bore of 1 and stroke of 2. It has poppet valves for the inlet and exhaust which are operated by two cams, one for each valve. Supplementary hand operated cams are used to hold the valves open for warming through. The crankshaft is mounted vertically with a spline machined on the end to match that on the drive shaft of the leg. By using the forward/reverse gears in the gearbox, there is no need for reversing gear on the engine. The valve chests and valves are from stainless steel with most of the rest of the engine being aluminium. The boiler is a monotube comprising 30 metres of stainless steel tubing, half of which is 3/16 and the rest is OD. A jig was made to facilitate the bending of the tube into grids, which were then assembled by TIG welding with all the joints on the outer edges of the assembly. The boiler fits into a stainless steel casing 10 square x 20 tall, lined with Kaowool refractory. There is a grate for wood firing and a fan for forced draft. The boiler sits on a frame in front of the engine which doubles as the steering tiller. The assembly weighs around 45 kg. A damper is fitted to the fan inlet to control the pressure. The boiler operates at 500 p.s.i. and 400 degrees C and the damper is fully closed when these conditions are met. There are two feed pumps driven by a toothed belt at 1/5th engine speed and using a disc crank scotch yolk. This has four positions for the crank pin allowing adjustment to give the correct amount of water to replace the steam used by the engine. Cylinder lubrication is by a model loco type ratchet pump, and it is planned to use colloidal graphite instead of oil to avoid contamination of the condensate with oil. A condenser is fitted, and an air pump is driven from the scotch yolk. The condenser is a length of copper pipe passing down the leg into the water next to the propeller

Update 21 June 2004: The graphite lubrication was not a success, the reason being that as soon as the aqueous suspension of graphite got close to the superheated steam it dried up, leaving the graphite to block the feed pipe. It also had a tendency to dry in the pump when the engine was not being used.


Update 19 September 2006

 The last  two photos are of  the new outboard and the dinghy which were taken during trials.  The chap in the bow is Brian Forster who owns the steamboat "Lady Lyn" and I am at the helm.

The other photos shows the unit out of the water and one can see the boiler mounted above the engine. The engine is a 90 degree vee twin, double acting, with  poppet valves so as to use the superheated steam from the monotube boiler. The crankshaft of the engine is vertical and it has a splined socket in its end to mesh with the splined shaft of the leg which was taken from a Mariner two stroke outboard. The two copper pipes which extend down from the cylinders down to near the propeller are the condenser. Feed water is carried in a tank inside the boat from which the feed pump draws to feed the boiler and the condensate is returned to this tank. Fuel is wood, I use broken garden stakes cut into 100mm lengths. Light up is done with newspaper and a few twigs, once the engine is running air for combustion is forced into the boiler furnace by an engine driven fan.  This fan has a choke in its inlet which closes when the steam pressure reaches 400 psi, this controls the steam pressure to a max of about 1000 psi. Since the boiler is all 1/4 inch OD stainless tube and contains a minimal amount of water it is very safe. The feed pump is driven at 1 fifth of the engine speed, it is a  reciprocating pump operated by a scotch yoke and its stroke can be adjusted to give the optimum amount of water to control the steam temperature, the ideal steam temp. is 400 degrees C and this is monitored by a thermocouple pyrometer. There is also a reciprocating condensate pump. An oil pump for cylinder lubrication is driven by a worm gear at 1/200 th of the engine speed.

This is the second steam outboard which I have built and my motivation for building the second one was to reduce the weight of the unit.  The new one weighs 33 kg, 12 kg lighter than the first one. Weight reduction was achieved by re-designing the boiler so as to use a lighter casing and less refractory, the two cylinder engine is lighter than the original since it needs no flywheel. It also has the advantage that it can be balanced much better and will run faster without vibration. I have made the boiler easily detachable so that it can be taken off when the boat is being trailered, this reduces the shocks on the transom from driving on rough surfaces.




Click on these thumbnails for a larger view

Engine & Boiler unit

Close up of the engine

The Complete Package!